'SOME EFFECTS OF LOGGING AND SLASH BURNING E ON PHYSICAL SOIL PROPERTIES
IN THE CORVALLIS WATERSHED
RESEARCH PAPER NO. 19
PAC1 FlC NORTHWEST FOREST AND RANGE EXPERIMENT STATION R. W. COWLIN , DIRECTOR
U. S. DEPARTMENT OF AGRICULTURE FOREST SERVICE
M A Y 1957
SOME EFFECTS OF LOGGING AND SLASH BURNING
ON PHYSICAL SOIL PROPERTIES
IN THE CORVALLIS WATERSHED
BY
C. T. Dyrness and C. T. Youngberg Oregon State College, Department of Soils
and
Robert H. Ruth Willamette Research Center
Pacif ic Northwest F o r e s t and Range Ejrperiment Station
Pacific Northwest F o r e s t & Range Experiment Station U. S. Department of Agriculture Fores t Service
Portland, Oregon May 1957
Foreword
This study was conducted by C. T. Dyrness as part of the requirements for the Master of Science degree, Oregon State College. Dr. C. T. Youngberg of the College Is Department of Soils, provided supervision and guidance for the study. Robert H. Ruth of the Pacific Northwest Fores t and Range Experiment Station's Willamette Research Center (maintained a t Corvallis, Oregon in cooperation with the School of Forestry, Oregon State College) joined in planning the project and in preparation of this report. The study was conducted on the Siuslaw National Forest .
Mr. Dyrness did the field work and laboratory analyses during the 1955-56 school year. He was assis ted by a contri- bution of funds from the Division of Watershed Management Re- search, Pacific Northwest Fores t and Range Experiment Station.
R. W. Cowlin, Director
SOME EFFECTS OF LOGGING AND SLASH BURNING
ON PHYSICAL SOIL PROPERTIES IN THE
CORVALLIS WATERSHED
C. T. Dyrness, C. T. Youngberg, and Robert H. R u t d l
INTRODUCTION
Many persons question the wisdom of harvesting t imber in forested drainages that supply municipal water. In the Douglas -fir region of the Pacific Northwest there a r e examples of logging that support this viewpoint but others a s well that tend to refute i t , Some municipal water supplies have been damaged and yet in other cases relatively l i t t le h a r m has been done.
No unqualified statement can be made about logging in municipal watersheds. Much depends upon physical conditions characterizing each individual a r e a . One of the mos t important of these i s soil and the way i t reac ts to disturbance.
This is a study of three soil types following logging and s lash 21 burning in a watershed supplying the city of Corvallis, Oregon. -
Until recently the fores t cover in this watershed, chiefly ma tu re Douglas -fir , remained undisturbed. In 1951 a seve re windstorm followed by a Douglas - f i r bark-beetle epidemic sharply increased the normal ra te of mortali ty, leaving patches of dead t r e e s totaling
11 The authors wish to express their appreciation to R. G. ~ e t e r ~ o n and L. D. Calvin of the Oregon Agricultural Experiment Station f o r guidance in s tat is t ical analysis of data used in this report .
21 The Corvallis watershed includes Rock and Griffith Creeks, both fl<wing f rom the eas tern slopes of Marys Peak in the Oregon Coast Range. The watershed a r e a used in this study is entirely within the Siuslaw National Forest.
roughly 60 million board-feet. In 1953 a logging program was begun to salvage some of this timber and to reduce the serious f i re hazard created by concentrations of dead material. In three years, 23 miles of road have been constructed and timber worth one-half million dollars in stumpage value has been salvaged from areas where beetles had centered their attack. Removal of these concentrations has resulted in a scattering of small clearcut areas .
b
PURPOSE AND SCOPE
This study was designed to evaluate some physical properties of the soil following logging and slash burning. It is not an attempt to assess the f u l l effects of these two types of disturbance. Measure- ments were limited to properties of soil frequently associated with erodibility: content of sand, s i l t and clay, aggregation, and per cent - ages of organic matter . A combination of these factors, plus moisb ,
ture equivalent determinations, was used to arr ive at an index of erosion.
Investigations were limited to three cutover a reas with differ- ent soil types. Results a r e generally applicable to other = i l s belong- ing to the Reddish-Brown Latosol great soil group in the Oregon Coast and Cascade Ranges. However, variables such a s soil mois- ture condition, slope gradient, aspect, and method of logging will also influence reactions to logging and slash burning in any given location.
OTHER STUDIES
Other workers in the Douglas -fir region have explored the influence of logging and slash burning on soils, using somewhat different approaches to the problem. Recently, Tarrant studied the effect of slash burning on ra te of water percolation, size of pore spaces, and bulk density of soils in two separate areas..?/ He found that severe burning lowers the rate of moisture movement, while light burning does not seriously al ter the soil in this respect. He showed further that severe burning occurred on less than 5 percent of the total logged and burned a r ea included in his study.
3 / Tarrant, Robert F. Effect of slash burning on some physicTl soil properties. Fores t Sci. 2:18-22, illus. 1956.
Another study dealing with disturbance to soil by tractor log- ging in the Douglas-fir region was reported in 1955 by Steinbrenner and ~ e s s e l . 4 ~ They were interested in compaction as measured by permeability rates, bulk density, and pore space. Soils of nine cut- over areas logged by tractors in southwestern Washington, when compared with adjacent uncut timber stands, showed a 35 percent lass in permeability rate, a 2.4 percent increase in bulk density and a 10 percent reduction in macropore space. Tractor skid roads covered 26 percent of the logged areas. Their contribution to total disturbance i s indicated by the results from tractor roads alone. Here permeability rates were reduced 93 percent, bulk density in- creased 15 percent, and macroscopic pore space reduced 53 percent.
METHODS
In the Corvallis water shed, three selected cutting units, logged with a high-lead system followed by slash burning, were sampled during the spring and fall of 1955 to determine :
1. Extent and distribution of recognizable surface soil conditions in each clearcut unit.
2. Some of the characteristics of soils associated with these surface conditions,
The three clearcut units examined vary in size from 10 to 27 acres (figs. 1, 2 and 3). They a re described a s follows:
Cutting unit Logged Burned Soil description
3 1953 1953 Blachly loam
4 1954 1954 Melbourne clay
10' 1954 19 5-5 Olympic clay
Although the burns were of different ages, the effects of f ire were readily apparent in all three units.
4/ Steinbrenner, E. C., and Gessel, S. P. The effect of tractor logging on physical properties of some forest soils in south- western Washington. Soil Sci. Soc. Amer. Proc. 19:372-376. 1955,
Figure 2. --Clearcut unit 4 in July 1956, two years after logging and slash burning. Soil i s Melbourne clay. (F. S. Photo 481318)
Figure 3. --Clearcut unit 10 in July 1956, two years after logging and one year after slash burning. Soil is Olympic clay. (F. S. Photo 481322)
Figure 4. --Timber stand and understory vegetation in undisturbed condition adjacent to unit 10.
Soil samples to a depth of two inches were collected at 50-foot intervals along randomly located transects within the clearcut a reas and in adjacent uncut timber (fig. 4), Condition of the surface soil was noted at 10-foot intervals along the transects in the clearcuts and classified a s follows : (1) lightly burned, where the surface lit ter is charred by the f ire but not completely removed; (2) severely burned, where the lit ter has been completely removed and the min- era l soil left with a brick-red color; ( 3 ) disturbed-unburned, where there is evidence of disturbance during logging but no visible effect of f ire; (4) undisturbed, where surface condition i s identical to that found in undisturbed timber, having no evidence of disturbance either by logging or by fire. This classification system is similar to that described by ~ a r r a n & except for the addition of the disturbed- unburned category.
From the 80 to 90 soil samples collected in each clearcut, 60 were randomly selected for laboratory analysis. This allowed 12 samples for each of the five soil conditions (undisturbed, disturbed- unburned, lightly burned, severely burned, and uncut timber). Lab- oratory determinations were made for each of the 180 soil samples as follows :
1. Mechanical analysis to determine content of sand, silt, and clay by a modified Bouyoucos hydrometer method.
2 . Moisture equivalent determinations by the standard Briggs -McLane centrifuge method.
3. Aggregate analysis, following the procedure a s out- 6 / lined by Yoder.-
4. Total organic matter determinations by the rapid dichromate oxidation method (modified by Walkley and Black).
5/ Op. cit. - 6 / Yoder, R. A direct method of aggregate analysis of soils
and az tudy of the physical nature of erosion losses. Agronomy Jour. 28:337-351, 1936,
With the exception of the aggregate analysis, all of the above deter- minations were carried out according to the procedures adopted by
7/ the Fores t Soils Committee of the Douglas-fir Region. - RESULTS
Almost half the a rea in the three clearcuts remained untouched by f i r e (fig. 5). Most of the remainder was classed a s lightly burned, leaving eight percent of the a r ea in the severely burned category.
Results varied considerably between the three clearcuts. It was found, for example, that the percentage of severely burned soil surface ranged from 1 . 3 percent on unit 3 to 16.1 percent on unit 10. As the graph shows, the slash f ire on unit 3 resulted in a very mild burn, while unit 10 burned much hotter than average.
Mechanical analysis of the 180 soil samples revealed no con- sistent change in soil texture a s a result of logging and burning for most of the a rea within the clearcut units (table 1). In the Blachly loam of unit 3 , statistical analysis indicated no significant difference in texture of the soils in the unit and under adjacent timber. This was true even for severely burned soil. In units 4 and 10 neither logging disturbance nor light burning had important effects on the soil texture classes measured. Severe burning, however, was associated with a significant reduction in the clay and silt -plus - clay fractions and an increase in sand in the top two inches of soil.
Extreme heat may explain the alteration of soil texture in the severely burned areas of units 4 and 10. Increase in sand accom- panied by the loss of s i l t and clay suggests the fusion of clay particles. It would be reasonable to assume that the resultant newly formed particles would be very stable a s the result of complete dehydration. These products of heat fusion apparently were so resistant to dis- persion that they were included in the portions described a s sand.
The sandier soil in unit 3 was affected differently by severe burning than were the clay soils of other units. It i s probable that the aggregates resulting from clay fusion were relatively small and silt-sized. Evidence for this is the lower clay percentages and the increase in silt content of severely burned samples.
7/ Forest Soils Committee of the Douglas-fir Region. Sam- pling procedures and methods of analysis for forest soils. Col. of Forestry, Univ. of Wash. 38 pp. (Processed. ) 1953.
PERCENT OF TOTALCLEARCUT AREA
No. 3 No. 4 No. 10 All
CLEARCUT UNIT
LEGEND: 1-1 Undisturbed
Disturbed- Unburned
Lightly burned
Severely burned
Figure 5. - -Soil surface conditions following logging and slash burning in three clearcut units, Corvallis watershed.
Table 1. - -Clay and silt-plus -clay fractions in the top two inches
of soil in three clearcut units and under adjacent
timber stands, Corvallis watershed
Soil texture class : Clearcut unit : Unit & surface condition : 3 4 10 : average
Clay (2 microns) - - - - - - - * - - - - - - - Percent - - - - - - - - - - - - - -
Timber 27.0 45. 8 47.6 40. 1
Undisturbed 25 .2 48.3 49 .4 41.0
Lightly burned 25.2 42.3 46. 1 37,9
Severely burned 23. 8 L I 3 9 . 9 1/37. 8 "33.8
All conditions 25. 1 4 4 . 4 45.4 38. 3
Silt -plus -clay
Timber 55.8 70.8 83.3
Undisturbed 53.9 72.0 85.2 70 .4
Disturbed-unburned 53.1 69.0 84.4 68, 8
Lightly burned 56. 7 66.7 84.2 69.2
Severely burned 57.9 A I 6 2 . 1 L'66.6 , , 62.2
All conditions 55.5 68. 1 80.7 68. 1
1 / Decrease significant a t 1 percent probability level. -
Organic matter in the -top two ipchesr of soil was essentially the same in the clearcuts a s in the check areas, except for the severely burned soil (table 2). Severe burning reduced organic matter to about 40 percent of the amounts in soil under the timber.
It was also found that severe burning lowered the level of total soil aggregation (table 3). On the remaining 92 percent of the clear- cuts, however, there was- no s ignificant difference in soil aggregation from the check areas .
Table 2. --Percentage of totalxmganic matter in the top two inches
of soil in three clearcut units -and under adjacent
kimber stands, Corvallis watershed
Soil surface Clearcut unit : Unit condition t . 3 (I 4 : 10 : average
Timber
Undisturbed 12. 6 8. 6 12.2 11.1
Disturbed- unburned
Lightly burned 12.3 5/12: 3 ' 10.4 11,7
Severely burned 1/ 3.0 . k/ 5.2 . 1' 4.5 1 4. 2
All conditions 10.6 ! 9.1 9 . 7 ' 9.8
1 / Decrease significant a t 1 percent p r~habi l i ty level. - 21 Increase significant a t 5 percent probability level. -
Table 3. - -Percentage of aggregates (0.1-5.0 mm) in the top two
inches of soil in three clearcut units and under
adjacent timber stands, Corvallis watershed
Soil surface condition : Clearcut unit : Unit 3 4 10 : average
Timber 34. 9 44.1 79.1 52.7
Undisturbed 36, 1 49.6 78. 6 ~ 5 4 . 8
Disturbed- unburned 33.5 41. 9 72.8 49.4
Lightly burned 35.1 39.1 77.7 50.6
Severely burned 29. 1 1' 30.4 1'65.8 1'41.8
All conditions 33.7 41.0 74.8 49.9
1/ Decrease significant a t 1 percent probability level. -
Lowered aggregation in the severely burned soils may be largely due to the removal of colloidal organic matter by fire. Many aggregates in surface soils depend on colloidal organic matter a s their cementing agent and i t s loss leads to disintegration. Although fusion of particles appears to occur a t high temperatures, this proc- ess apparently fails to off set breakdown of naturally formed aggre- gates which is occurring simultaneously.
Results of the moisture equivalent determinations likewise indicat+ed that the ability of the surface soil to retain moisture is sig- nificantly lessened by exposure to intense heat (table 4). Other treat- ments had no consistent effect on moisture equivalent values in this study. Reduced moisture equivalent values in severely burned soils may be attributed to the slightly coarser texture they acquire and to loss of organic matter .
Table 4. --Moisture equivalent values for the top two inches
of soil in three clearcut units and under adjacent
t imber stands, Corvallis watershed
Soil surface condition : Clearcut unit : Unit 3 4 : 10 : a v e r a g e
Timber 34. 6 41. 1 45.7 40. 5
Undisturbed 35, 0 40. 9 46. 5 40. 8
Disturbed- unburned
Lightly burned 36.3 38.4 45.7 40.1
Severely burned 32.6 L1 36. 3 1' 36.9 35. 3
All conditions 34.4 39.1 43.6 39.0
1 / Decrease significant a t 1 percent probability level. -
EFFECTS ON ERODIBILITY
In considering the possibility of erosion in any a rea , f i r s t need is to determine the inherent erodibility of the soil itself. This in- formation can be obtained prior to logging and can be very useful in prescribing precautions to be taken.
An analysis of undisturbed soils adjacent to the clearcut units in the Corvallis watershed revealed that the soils a r e naturally quite resistant to erosion. They have a high percentage of water-stable aggregates attributable to generous amounts of colloidal clay, organic mat ter and iron oxides. Although no measurements were taken, the porous and granular nature of the surface soil suggests a rapid infiltration rate and a minimum of runoff, even during heavy rains.
Erosion ratios were determined for soil samples f rom the severely burned and lightly burned areas of cutting unit 10 a s well a s from the adjacent t b b e r . The initial s tep was to determine the re - sistance of si l t and clay to dispersion. Each sample was shaken in water to obtain a measure of aggregate stability, which was expeessed a s a dispersion ratio. This index of dispersion was then divided by a ratio of clay content to moisture equivalent, which gave the erosion ratio. Middleton, who originated the procedure, maintains that erosion- ratio values a r e an accurate index of the relative erodibility of the soi1.Y Mean ratio values obtained for the three soil conditions were a s follows: timber, 3.01; lightly burned, 4.19; and severely burned, 9.97.
Middleton found that soils having an erosion ratio greater than 10 were most frequently susceptible to erosion and those with a smaller ratio were virtually nonerodible. Mean values for the above three conditions would indicate that the ratio increases with the degree of burn. Still, erodibility of the soil is not seriously increased by burning, a t least insofar as i t is assessed by this method.
Perhaps one of the most important factors contributing to the low rate of soil erosion in the Corvallis watershed is the pattern of rainfall. Total precipitation is high but--in a manner typical of the Douglas -fir region- -it usually falls a s light, gentle rain and quickly enters the soil. Miltrat ion capacity of the soil mantle is seldom surpassed; consequently the amounts of surface runoff and soil ero- sion a r e small.
Results of this study show that light burning had very little effect on the physi cal soil properties that were investigated. In each case results obtained f o r lightly burned soils were not signifi- cantly different f rom those for undisturbed sods .
Physical properties of the surface soil located in the disturbecl- unburned portions of the clearcut showed evidence of some alteration, due primarily to compaction. This occurred mainly on skid trai ls which comprised a large portion of the a r e a classed a s disturbed- unburned in the average clearcut unit. Structural breakdown from compaction may have been responsible for lowering of both moisture equivalent and percentage aggregation values .
8/ Middleton, H. E. Propert ies of soils which influence soil - erosion. U. S. Dept. Agri. Tech. Bul. 178, 16 pp. 1930.
Results from undisturbed soils agree closely with those in adjacent undisturbed timber. The undisturbed parts of the clearcut a r e apparently still a s resistant to erosion as the soil located in the surrounding timber. Presence of a vegetative cover and a thick layer of li t ter has, so far , prevented the disintegration of structure needed to maintain rapid infiltration rates.
Surface lit ter is a very important factor in the control of soil erodibility. Severely burned a reas were all completely bare of organic l i t ter , thus exposing the soil surface to the beating and de- structive action of rain. With no organic layer to absorb the impact of falling rain drops, aggregates a r e broken down, soil thrown into suspension, and the natural porosity of the soil quickly diminished. This, in turn, causes a decrease in permeability of the soil, and more runoff and erosion result.
Several factors, however, tend to minimize the danger of soil erosion in severely burned a reas :
1. Often a crust was present on the soil surface of the severely burned areas . It appeared that this crust provided some protection against soil movement.
2. The severely burned soil condition occurred almost entirely in small scattered areas. This tended to reduce the importance of the increased erodibility.
3 . Very few severely burned areas were located on steep slopes where erosion would be more apt to occur. Most. were found on gentle slopes and benches where topography was more conducive to slash accu- mulation,
CONCLUSIONS AND DISCUSSION
Physical soil properties measured in this study did not differ significantly among 4 of the 5 surface conditions sampled. Only in the severely burned condition was there found a consistent and sig- nificant departure, indicating that intense heat altered the character of the surface soil. Physical properties of soil in undisturbed, dis- turbed-unburned, and lightly burned portions of the clearcuts re- mained closely similar to those under adjacent timber stands.
Severe burn, the one surface condition where soil was seriously affected, occupied only a small portion of the cutover a r ea s in the watershed. In this study i t involved an average of only 8 percent of the three units examined. This i s probably higher than the average fo r the Douglas-fir region, a s indicated by Tarrant 's measurements. In unit 10, where 16 .1 percent of the a r e a was classed a s severely burned, the slash f i r e was exceptionally hot.
The amount of severely burned soil present following broadcast slash burning depends largely upon three factors: (1) the amount of slash to be burned, (2) i ts distribution, and ( 3 ) the moisture status of the slash, l i t ter , and soil a t the time of burning. If, for example, the slash is well distributed over the entire a rea , the danger of ex- posing the soil to extremely high temperatures in localized a reas i s lessened considerably. Furthermore, the level of wood utilization affects the amount of the severely burned soil condition. It i s to be expected that in the future i t will be economically feasible to utilize many of the logs that a r e now rejected. Since many of the severely burned a r ea s resulted from prolonged f i r e in cull logs, increased utilization will decrease the occurrence of the severely burned soil condition.
In recent years i t has been the practice to burn soon after a fall rain. Burning the slash as soon a s i t dr ies out but when the l i t ter and soil a r e sti l l moist results in fa r l e s s damage to the soil. Severely burned soil is limited to a r ea s immediately beneath large accumula- tions of slash, and even here the effects of the f i re penetrate the so il to a limited depth. Spring and ear ly summer burning have the same effect i f soils a r e moist.
This study can be considered only a partial evaluation of the effects of logging and slash burning on soils in the Douglas-fir region. Soil tests were not intended to be a oo mplete investigation of the soil changes which occur. Furthermore, only three types of soil were examined. These a r e known to occur throughout the Coast and Cas- cade Ranges in Oregon and Washington, so, while limited in scope, resul ts of this study a r e applicable over a substantial part of the Douglas-fir region. Still, to a s s e s s fully the effect of logging and slash burning, other soils associated with the Douglas -fir type will need to be sampled and subjected to comprehensive tests .
